Receiver for spectrum analysis

ABSTRACT

A receiver for spectrum analysis includes at least one mixer in which a radio frequency input signal to be measured is mixed with an oscillator frequency generated by a voltage-controlled oscillator. In a detector, the level of an input signal to be measured is detected, after the signal has been transferred onto a lower frequency. The use of a logarithmic amplifier may be avoided by directing the detected signal to a decision means, which, in response to a change in the level of the detected input signal, provides a control voltage proportional to the magnitude and the direction of the change. The voltage commands an adjustable attenuator to attenuate the signal to be measured so that the level of the detected input signal substantially remains constant. The attenuation of the attenuator measured in decibels is linearly dependent on the control voltage, so that the values of the control signal directly represent the amplitude values.

This application is the national phase of international applicationPCT/FI96/00037 filed Jan. 17, 1996 which designated the U.S.

FIELD OF THE INVENTION

This invention relates to a receiver which measures a spectrum of aradio-frequency input signal. The receiver includes a local oscillatorsignal generator, a mixer in which the input signal and the localoscillator signal are mixed to produce an intermediate frequency signal,and a detector which detects an envelope of the input signal that hasbeen transferred onto the intermediate frequency.

BACKGROUND OF THE INVENTION

In spectrum analysis, energy distribution of an electric signal isstudied as a function of frequency of a spectrum analyzer, which allowsgraphic representation of the amplitude as a function of frequency in aportion of the spectrum. The analyzer may be used as a sensitivereceiver to measure attenuation, FM deviation, and frequency, as well asto study RF pulses. A simple spectrum analyzer is based on asuperheterodyne receiver and an oscilloscope. In the receiver, the inputsignal is mixed in the mixer with a frequency obtained from avoltage-controlled oscillator. The control voltage of the oscillator isa saw-tooth voltage, whereby the frequency of the oscillator sweeps overa certain frequency range. An intermediate frequency signal, obtained asthe mixing result, is amplified and detected. The detected signal, whichis amplified in a video amplifier, is directed to the verticaldeflection plates of a cathode-ray tube and the saw-tooth voltage isdirected to the horizontal deflection plates. As a result, an amplitudeis shown on a display as a function of frequency. In older spectrumanalyzers, the detector and the video amplifier were combined in thesame block.

FIG. 1 shows a simplified block diagram of a prior art spectrum analyzerprimarily intended for monitoring. An incoming RF signal is attenuatedin an attenuator 1, whereafter it is low-pass filtered in a filter 2before mixing it in a mixer stage 3 with a oscillator frequency f₂. Theoscillator frequency f₂ is generated in a YIG oscillator block A, whichcomprises an oscillator 4 and its control circuits 5, 6. The frequencyf₂ of the YIG oscillator varies as a function of a ramp voltage obtainedfrom a ramp generator 7, whereby the oscillator frequency sweeps over adesired frequency band. The horizontal deflection voltage of acathode-ray tube CRT changes in response to a change in the voltage ofthe ramp generator 7. From the mixing results obtained in the mixer 3, adesired frequency is filtered in an adjustable band-pass filter 8,whereafter the signal frequency is further lowered in two successivemixer stages 9, 10 using fixed oscillator frequencies. The resolutionband may thus be made narrower. After amplification carried out in achain of switchable intermediate frequency amplifiers 11, band-passfiltering performed in a filter 12 with an adjustable pass band, andamplification carried out in an intermediate frequency amplifier 13, thesignal is applied to a block 14 for performing a logarithmic conversion.In this block, detection is also performed. Combined component packagesthat carry out detection and logarithmic amplification are commerciallyavailable. Block 14 provides as an output the amplitude variation of thedetected signal as a function of frequency and in accordance with thelogarithmic scale. The vertical deflection voltage of the cathode-raytube varies in response to the output of block 14, whereby the amplitudeof the input signal is drawn on the tube as a function of frequency inthe decibel scale.

In prior art spectrum analyzers, a video amplifier is employed. The nameoriginates from a detector in which a logarithmic amplifier, that is, achain of saturated amplifiers with a limiter output is used after orbefore envelope detection. The precision of a video amplifier of thiskind is high, but its cost is high. An IF amplifier circuit with an RSSI(Received Signal Strength Indicator) circuit at the output may also beused. The advantage of amplifiers applying this solution is a low cost,but the drawback is low precision. The dynamic range that may beachieved with prior art analyzers is some 60 dB, which limits the usesof the analyzer. Furthermore, video amplifier circuits may be verycostly depending on the qualities, and the variation between analyzerscaused during the production is rather great, and adjustments ofdifferent kinds must thus be carried out for reducing the variation. Thethird drawback is caused by the fact that costly oscillator solutions,such as a YIG oscillator, must be used, since the requirements for theprecision of an adjustable oscillator are high.

The object of this invention is to achieve a receiver for spectrumanalysis, suited for analyzing a radio frequency signal, and to avoidthe drawbacks of the prior art receivers. The object is specifically tomeasure the signal level as a function of frequency in a simple andreliable manner, without using a logarithmic amplification chain, andachieve a wide dynamic range, up to 100 dB.

The object is achieved with a spectrum analyzer comprising at least onemixer, a detector, an attenuator, a decoder, and a controller andrecorder.

SUMMARY OF THE INVENTION

The receiver of the invention comprises an adjustable attenuator, agenerator which generates a control signal to adjust its attenuation inresponse to the level of a detected signal so that the level of thedetected signal remains unchanged, and a recorder which records thevalues of the control signal. In such a case, the values of the controlsignal represent amplitude values.

In accordance with a preferred embodiment, the receiver comprises amicroprocessor in which the values of the control signal are recordedand which processes the values to be displayed on a display unit. Themicroprocessor may also calibrate the adjustable attenuator. It may alsobe informed if the signal exceeds the dynamic range, whereby theoperation of the receiver continues in a predetermined manner.

In accordance with a preferred embodiment, the oscillator frequency isproduced by a phase-locked loop, the division values of its dividersbeing provided by the microprocessor, whereby it is possible to generatethe frequencies of the frequency range to be swept in desired frequencysteps, and the frequency information, as well as the value of theattenuator control signal proportional to the amplitude value of thedetected signal are recorded in the microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be disclosed in greater detail bymeans of a preferred embodiment of the invention with reference to theattached drawings, in which

FIG. 1 shows a block diagram of a prior art receiver for spectrumanalysis and

FIG. 2 shows a block diagram of a receiver for spectrum analysis inaccordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the receiver of the invention, an incoming RF signal RFIN to bestudied is mixed with the oscillator frequency f₂ in a mixer 230 ontothe intermediate frequency of a band-pass filter 280, whereafterdetection is carried out in a detector block 214 as shown in FIG. 2.There is no logarithmic amplifier after the detector as in prior artsystems, and the level of the detected signal is studied in a detectionblock 215. An attenuator 210 attenuates or amplifies the input signal inresponse to the information provided by block 215 so that the level ofthe detected signal remains constant. The principle is similar to anAutomatic Gain Control (AGC) applied in radio receivers.

When the oscillator frequency f₂ is swept over a desired band and theadjustment values of the attenuator 210 are recorded in the memory, itis easy to form a spectral curve in the display unit by processing theadjustment values since the adjustment values are proportional toamplitude values.

In the following, the operation of the receiver is to be discussed inmore detail with reference to FIG. 2.

An RF signal to be measured, having a frequency f₁, enters a filter 211,which limits the useful bandwidth of the signal to be measured withinthe right range. Thereafter, the signal is attenuated or amplified in anadjustable attenuator 210 so that the level of the detected signalremains constant after detector 214 at point B. The adjustableattenuator 210 may be implemented in the form of a chain of PIN diodesor a commercially available attenuator microcircuit. An importantrequirement for the attenuator is that its attenuation in decibels,depends linearly on the control voltage V_(CNTRL), i.e., the change V/dBis linear. A suitable change is, e.g., 1V/10 dB. For achieving thedesired change in practice, it is necessary to use a number ofattenuation stages, whereby the calibration voltage V_(Calibr). may beapplied to one attenuation stage. The calibration voltage is obtainedfrom a microprocessor 232 via a D/A converter 219 in a manner disclosedhereinafter.

The frequency f₁ of the signal to be measured, attenuated/amplified inthe attenuator 210, is transferred onto a lower frequency by mixing itin the mixer 230 with a oscillator frequency f₂ signal, which isobtained from a voltage controlled oscillator 217. In accordance with apreferred embodiment, frequency f₂ is generated by a phase-locked loopPLL. The phase-locked loop includes the voltage controlled oscillator217, a phase comparator, a loop filter, a reference oscillator, anddividers. Integrated circuits including the components, of the PLLcircuits, are commercially available. As is generally known, aphase-locked loop forms a frequency synthesizer in a case where itsdivisors may be changed by software. The frequency to be synthesizedwith the loop, the oscillator frequency f₂, is determined by themicroprocessor 232 by feeding suitable divisors to a programmable loopdivider (not shown) of the PLL block 231. When the divisors are changed,the control voltage V_(VCO) of VCO 217 changes, and the frequencygenerated by the VCO may be changed gradually over a desired frequencyband. By generating the oscillator frequency with a phase-locked loop,excellent frequency precision and a remarkably lower price are achievedin the narrow frequency range used as compared with the YIG oscillator,for instance.

An intermediate frequency signal is filtered from the mixing results ina band-pass filter 280. The filter limits the resolution to be measured.A signal to be measured, after being transferred onto a lower frequency,is directed to the detector 214, the output of which provides theenvelope of the signal. Thereafter, the detected signal is directed to adecision block 215, in which it is studied whether the level of theinput signal is higher or lower than the values given at point B. If thelevel is higher or lower than the given values, the decision block 215provides a control signal V_(CNTRL), which commands the attenuator 210via a low-pass filter 216 to attenuate or amplify the input signal RFINso that it remains on the desired level at point B. The value of thecontrol signal V_(CNTRL) represents the position of the attenuator 210,and that is exactly the information desired, because the verticaldeflection of the display unit (not shown) is controlled according toit. In other words, the dB value of the signal may easily be calculatedbased on the value on a specific frequency. If such an attenuator isused the attenuation of which measured in decibels depends linearly onthe control voltage V_(CNTRL), calculation is simple. The value of thecontrol signal V_(CNTRL), which is the desired information, is convertedinto a digital form in the A/D converter 218 and recorded in themicroprocessor 232. The processor also has a corresponding frequencyinformation owing to the fact that it has set the oscillator frequencyby feeding the divisors into the phase-locked loop. Therefore, frequencyinformation (X coordinates) and dB information on attenuation (Ycoordinates) are recorded in the processor, and the spectrum may thus bedisplayed in the display unit either immediately or later. The recordeddata may also be processed easily either in real-time or bypost-processing.

The decision circuit 215 may be a comparator or an operational amplifierthat compares the output of the envelope detector 214 with the referencelevel, or it may be an intermediate frequency circuit supplied with anRSSI detector, whereby the RSSI voltage of the circuit is maintainedconstant by adjusting the attenuator 210. A prior art filter used inspectrum analyzers may be used as a low-pass filter suitable forfiltering the control voltage V_(CNTRL), which filter narrows the noisebandwidth of the signal entering the display of the spectrum analyzer,in this case the noise bandwidth of the signal entering the A/Dconverter 218.

The decision circuit 215 also provides envelope information, which isapplied to the comparator 233. When the level of the envelope exceeds agiven reference level, the comparator 233 provides the microprocessor232 with information the reference level has been exceeded, indicatingthat the dynamic range of the receiver has been exceeded, i.e., thesignal to be measured goes beyond the measuring range. In response tothis information, the microprocessor carries out a predeterminedfunction, that is, it gives an alarm, for instance. It would also bepossible to obtain the above-mentioned information from the controlvoltage information V_(CNTRL), that is, the information that theattenuator is in its extreme position, and attenuation thus cannot beincreased any more. It is remarkably faster, however, to act on theinformation provided by the envelope.

It is also possible to program a calibration table for themicroprocessor 232. The table comprises loading information of a PLLcircuit 231, that is, information on the frequencies generated by it,and information on the level error correction value, indicating how muchthe position of a calibration attenuator, which is a part of theattenuator 210, must be adjusted to achieve the correct value of theinput level. Calibration control V_(Calibr). is converted into an analogform in the D/A converter 219 and applied to the attenuator 210.

Calibration is carried out in the manufacturing of the receiver on theproduction line. It is also possible to implement a receiver arrangementin which an input gate is automatically connected to a calibrationgenerator, which may be a comb generator or a noise generator, and themicroprocessor 232 carries out the calibration phase by studying theinput level as a function of frequency, and produces correction valuesof the information.

If the filter 211 of the input signal of the receiver and/or the filter280 of the mixing results are voltage-controlled filters, the controlvoltage V_(VCO) of the VCO 217 may be used to adjust these filters,whereby the passband of the filters may be shifted in accordance withthe signal frequency to be studied. This possibility is shown in FIG. 2with control lines leading to the filters.

By a receiver for spectrum analysis of the invention, a simple andreliable measurement of the level of the signal is accomplished as afunction of frequency without a costly logarithmic detector. It shouldbe understood that the above explanation and the illustrationsassociated therewith are only intended to illustrate the presentinvention. Different variations and modifications of the invention willbe obvious to persons skilled in the art without deviating from thescope and the spirit of the invention set forth in the attached claims.Therefore, the detector and the decision circuit may be combined intoone commercially available circuit, such as an intermediate frequencycircuit supplied with an RSSI circuit. This is illustrated in FIG. 2with a broken line surrounding the detection and decision block. Themicroprocessor may also combine temperature calibration with thecalibration signal. In the case of the example, there is only one mixerstage, but, by increasing the number of the mixer stages, it is possibleto achieve very narrow resolution bands. It is also possible to employdirect mixing, whereby the band-pass filter 280 is replaced with alow-pass filter and an amplifier, that is, an active filter. The directmixing is, however, attended by the drawback that since the frequency ofthe VCO is the same as that of the input signal, the frequency of theVCO may leak into the input gate RFIN. The attenuator may also be placedafter the mixer at a suitable position prior to signal detection.

I claim:
 1. A receiver for spectrum analysis, said receivercomprising:at least one mixer, said mixer transferring a radio frequencyinput signal to be measured onto a lower frequency by mixing said inputsignal with an oscillator frequency generated by a voltage-controlledoscillator; a detector which detects a level of said input signal, aftersaid input signal has been transferred onto said lower frequency; anattenuator, said input signal being an input to said attenuator; adecider which provides, in response to a change in said level of saiddetected input signal, a control voltage, said control voltage beingproportional to a magnitude and a direction of said change, saidattenuator being adjustable, an adjustment voltage of said attenuatorcomprises said control voltage, said control voltage causing attenuationor amplification of an output signal of said attenuator so that saidlevel of said detected input signal remains substantially constant,whereby control voltage information is proportional to said level ofsaid input signal; and a controller and recorder which records andprocesses said control voltage information and oscillator frequencyinformation.
 2. The receiver as claimed in claim 1, wherein said decidercomprises a comparer which compares said level of said detected inputsignal with a reference level and wherein said control voltage isproportional to a deviation of said level of said input signal from saidreference level.
 3. The receiver as claimed in claim 1, wherein saidvoltage-controlled oscillator is a part of a frequency synthesizer andwherein said controller and recorder directs said synthesizer togenerate a desired oscillator frequency.
 4. The receiver as claimed inclaim 1, wherein said control voltage is directed to an A/D converter,whereby a digital value of said control voltage obtained from said A/Dconverter and directed to said controller and recorder is said controlvoltage information.
 5. The receiver as claimed in claim 1, saidreceiver further comprising a monitor which monitors a dynamic range,said monitor notifying said controller and recorder when said level ofsaid detected input signal exceeds a predetermined value.
 6. Thereceiver as claimed in claim 1, wherein said adjustable attenuatorcomprises a plurality of adjustable attenuation stages, said controllerand recorder adjusting attenuation of each of said stages by providingsaid stage with a calibration voltage.
 7. The receiver as claimed inclaim 3, wherein said control voltage of said voltage-controlledoscillator of said frequency synthesizer is an adjusting voltage for atleast one adjustable filter of said receiver.
 8. The receiver as claimedin claim 1, wherein said attenuation of said adjustable attenuator ismeasured in decibels and is linearly dependent on said control voltage.9. The receiver as claimed in claim 1, wherein said controller andrecorder is a microprocessor that processes said control voltageinformation and said oscillator frequency information so that anamplitude of said signal to be measured as a function of frequency maybe displayed.